`f(x) = sqrt(sin x - cos x)` is defined if `sin x ge cos x`.
To solve this, we draw the graphs of y = sin x and y = cos x and see where the graph of y = sin x is above the graph of y = cos x. Since the period of both functions is `2pi`, we solve for `x in [0, 2pi]` and then generalize.
From the graph, `sin x ge cos x`, for `x in [(pi)/(4), (5pi)/(4)]`.
Generalizing, we get `x in underset(n in Z) uuu [(pi)/(4) + 2npi, (5pi)/(4) + 2npi].`
